1. Field of the Invention
The present invention relates to a mass spectrometer wherein a sample solution is ionized by an atmospheric pressure ionization ion source such as ESI (Electro-Spray Ionization), a multi-charge ion produced in the ion source is introduced into a mass spectrometer, and a fragment ion is produced by Collision-Induced Dissociation (CID) or Infrared Multi Photon Absorption Dissociation (IRMPD) and mass analyzed.
Particularly, the present invention relates to a method and a mass spectrometer wherein charge reduction of the sample ion is carried out by using an ion having an opposite polarity with respect to the sample ion, and a mass spectrum of the fragment ion which tends to be complicated in a case of a multi-charged ion is simplified and analyzed with higher sensitivity.
2. Description of Related Art
A mass spectrometer is a device in which mass-to-charge ratio (m/z, where m represents the mass of the ions and z represents the charge of the ions) of sample ions is directly measured with high sensitivity and high precision. In recent years, the scope of its application has expanded to analyses of peptides and proteins. The analysis of those biomolecules is expected to be applied to various fields ranging from medical diagnosis to the design of drugs for treating new diseases.
Ion trap mass spectrometers are widely used in many fields because they can perform many functions in spite of being compact in size.
In recent years, mass analyses of peptides, proteins and DNA, etc. has become very popular, which is largely due to development of ionizing methods of the ion trap mass spectrometer. Typical such methods are Matrix Assisted Laser Desorption Ionization (MALDI) and Electrospray Ionization (ESI).
MALDI is an ionization method mainly for generating single-charge ions when ionizing proteins, and it is compatible with Time of Flight (TOF) mass spectrometry. In ESI, biomolecules become multi-charge ions, which are ions wherein one molecule (mass: m) has multiple charges (number of charges: n). Because mass spectrometers analyze mass-to-charge ratio (m/z), each multi-charge ion is identified by its mass-to-charge ratio of m/n.
Multi Stage Mass Spectrometry (MS/MS) is a method which determines the structure of a biomolecule ion produced by the above ionization method using a mass analysis. Parent ions are dissociated by methods such as CID and IRMPD. A pattern of the fragment ion is determined by a mass spectrometer so that the structure of the parent ion is determined.
In many cases of analysis, the required sensitivity is less than a picogram (pg=10−12 g). Compared to the component to be analyzed, there are many disturbing components which can cause problems. Therefore, reduction of the disturbance or noise is essential. This noise is called chemical noise. The charged particles which give substantially the same m/z as that of the sample ions to be analyzed become chemical noise during actual analysis. Such chemical noise might comprise an ion having a lighter mass and a smaller number of charges or a heavy cluster having many charges.
One way to discriminate between chemical noise and a component to be analyzed comprises a method of charge reduction as shown in Analytical Chemistry vol. 68 (1996), page 4026 and Internal Journal of Mass Spectrometry and Ion Processes Vol. 162 (1997) 89. A mass spectrometer comprises an ion trap, which has a fluorocarbon negative ion source by glow discharge. A positive sample ion produced in an ESI ion source is trapped in an ion trap mass spectrometer and, further, a negative ion is introduced there. Both ions are captured by the ion trap and attract each other by attracting Coulomb force.
The m/z of a multi-charge ion whose charge is reduced by the ion-ion reaction becomes greater compared to the m/z before the ion-ion reaction. Since the change in the value of m/z of the ion to be analyzed by the ion-ion reaction can be clearly distinguished from that of a chemical noise, it is possible to eliminate the chemical noise.
On the other hand, it is proposed in Analytical Chemistry, Vol. 72, p. 899 (2000), that charge reduction by the ion-ion reaction be used to simplify a spectrum of a multi-charged fragment ion produced after the MS/MS analysis. Because of the charge reduction by the ion-ion reaction, the number of candidates of m/z values based on the same mass m is reduced. Therefore, it becomes easier to analyze the spectrum. Further, discrimination between a multi-charged ion having greater mass and a chemical noise in the smaller mass region becomes simple.
In the prior art, chemical noises are eliminated and an analysis of a spectrum is made easier by charge reduction. However, since a reaction of a sample ion and an oppositely charged ion is stochastic, charge reduction continues until the number of charges, of the sample ion becomes zero, or the sample ion becomes neutral. In this case, the sample ion escapes from the ion trap and, as a result, analysis sensitivity is degraded.